This specification includes the following acronyms:
APaccess pointBERbit error rateCCKcomplementary code keying (RF modulation)DSSSdirect sequence spread spectrumEIRPequivalent isotropically radiated powerERPeffective radiated powerFECforward error correctionFERframe error rateMIBmanagement information baseOFDMorthogonal frequency division multiplexingPBCCpacket binary convolution codingPHYphysical layerPLCPphysical layer conversion protocolPMDphysical medium dependentPPDUPLCP protocol data unitPSKphase shift keyingPSNIperceived signal to noise indicationRPIreceived power indicatorRSSIreceived signal strength indicatorSQsignal qualitySTAstation
The current IEEE standard 802.11 is entrusted with the task of providing interfaces, measurements, and mechanisms to support higher layer functions for efficient network management. Presently, the 802.11 standard has defined several physical parameters, none of which is completely suitable for network management purposes. One example of a measurable parameter is received signal strength indicator (RSSI), which is a reportable parameter for each received frame but is not quantified in the standards, and is not fully specified. The standards do include certain definitions in the context of RSSI, but it remains that RSSI poses certain limitations for use in network management since RSSI parameters from different stations (STAs) may not be uniformly defined and thus are not comparable.
A second suggested measurable parameter is the signal quality (SQ), which also happens to be an unquantized indicator of code synchronization, but is only applicable to the DSSS PHY modulation and is not applicable to OFDM PHY modulations. Yet another measurable parameter is the RPI histogram, which, even though quantized and specified, cannot make target measurements on any AP. RPI histograms measure channel power from all sources including the 802.11 sources, radars, and all other interference sources, which is not helpful for relying on the RPI histogram as a controlling parameter.
Current standards define received signal strength indication based mainly on measurement of AP signals:
(1) on the same channel, same physical layer, and same station; and
(2) on different channels, same physical layer, and same station.
Significantly, measurements involving different physical layers and the same or different stations, even though required, are not presently addressed in the standards.
Network management needs comparative PHY measurements for use in handoff decisions, for example. The following types of comparative PHY measurements are made.
1. To compare AP signals on the same channel, the same PHY, in the same STA.
2. To compare AP signals on the same channel, the same PHY, in different STAs.
3. To compare AP signals on different channels, the same PHY, in the same STA.
4. To compare AP signals on different channels, the same PHY, in different STAs.
5. To compare AP signals on different PHYs in different STAs.
6. To compare AP signals on different PHYs in the same STA. Comparative measurements are crucial to handoff decisions for Network Management.
RSSI, as currently defined, only addresses categories (1) and (3) above. The RSSI is a measure of the RF energy received by the DSSS PHY or the OFDM PHY. RSSI indications of up to eight bits (256 levels) are supported. The allowed values for RSSI range from 0 through RSSI maximum. This parameter is a measure by the PHY sublayers of the energy observed at the antenna used to receive the current PPDU. RSSI is measured during the reception of the PLCP preamble. RSSI is intended to be used in a relative manner, and it is a monotonically increasing function of the received power.
CCK, ER-PBCC: the 8-bit value of RSSI as described in 18.4.5.11.
ERP-OFDM, DSSS-OFDM, the 8 bit value is in the range of 0 to RSSI maximum as described in 17.2.3.2.
Some limitations of the RSSI indicator are: RSSI is a monotonic, relative indicator of power at the antenna connector, which indicates sum of desired signal, noise, and interference powers. In high interference environments, RSSI is not an adequate indicator of desired signal quality. RSSI is not fully specified: there are no unit definitions and no performance requirements (accuracy, fidelity, testability). Since so little about RSSI is specified, it must be assumed that widely variant implementations already exist. It is not possible to compare RSSIs from different products and perhaps not even from different channels/bands within the same product.
Although RSSI has limited use for evaluating AP options within a given PHY, it is not useful in comparing different PHYs. RSSI must be rescaled for DSSS and OFDM PHYs. RSSI is clearly not useable by network management for load balancing or load shifting and RSSI from one STA does not relate to RSSI from any other STA.